Method of applying fluorescent material in cathode-ray tube manufacture



July 22, 1941- c. H. BACHMAN 2,250,189

METHOD 0F APPLYING FLUORESCENT MATERIAL IN CATHODE-RAY TUBE MANUFACTURE Filed May 5, 1937 INVENTOR ATTORNEY Patented July 22, 1941 METHOD OF APPLYING FLUORESCENT MA- TEBIAL 1N QATHODE-RAY TUBE MANU- FACTURE Charles H. Bachman, St. Marys, Pa., assignor to Hynade Sylvania Corporation, Salem, Masa, a corporation of Massachusetts Application May 3, 1937, Serial No. 140,369

7 Claims.

This invention relates to fluorescent devices such as cathode-ray tubes and the like, and more particularly to methods and means for attaching the fluorescent material to the wall of the tube.

A principal object of the invention is to-provide an improved method of attaching a powdered fluorescent material to a glass support.

Another principal object is to improve the emciency of the fluorescent screen of a cathode-ray tube or the like.

A feature of the invention relates to a method of attaching a coating of a powdered fluorescent material to a glass body whereby the body may be heated for drying or other purposes without danger of pitting or spotting the'coating.

Another feature relates to a method of attaching a coating of a powdered fluorescent material to a glass body without employing as a binder a material which tends to react with the glass when the latter is heated.

Another feature relates to a method of attaching a coating of powdered fluorescent material to a glass body without employing a binder containing organic materials.

A further feature relates to a fluorescent screen which has improved secondary-electron emissive properties.

A further feature relates to a cathode-ray tube wherein the fluorescent screen contains a material or materials which enhance the ability of the screen to emit secondary electrons without rendering the screen liable to pitting or blackening during the heating or other processing thereof.

A still further feature relates to an improved cathode-ray tube or the floating fluorescent screen type.

Other features and advantages not specifically enumerated will be apparent after a consideration of the following detailed descriptions and the appended claims.

While the invention will be disclosed herein as embodied in one particular kind of fluorescent device, it will be understood that this is done merely for explanatory purposes and not by way of limitation. Furthermore only those parts of the device are illustrated as will enable the invention to be completely understood. Accordingly, in the drawing, there is shown diagrammatically one iorm of cathode-ray tube which may be, although not necessarily, of the structure disclosed in 0. H.'Biggs Patent No. 2,176,199, granted October 17, 1939. In general, this tube comprises an enclosing envelope I having an elongated neck 2 joined to a funnelshaped portion 3 terminating in a substantially flat end portion or window 4. The inside surface of window 4 is provided with a fluorescent coating 5 in accordance with the invention. Sealed into the neck portion is any well-known form of electron gun 6. Mounted between the gun and the screen are a plurality of electrodes of any known construction preferably comprising a con trol electrode or grid 1, a focussing electrode 8, an anode 9, a set of deflecting plates l0, and a secondary emission shield II. By this arrangement the electron beamis projected to, and is focussed on, the screen 5, and the beam is deflected in accordance with the desired pattern under control of the deflecting plates 10. In this arrangement it will also be noted that the screen 5 is floating, that is it is not conductively connected to any of the remaining electrodes. However the return beam current is carried by the secondary electrons that are emitted from screen 5 when bombarded by the beam. The return circuit for these secondary electrons is by way of the shield H which is conductively connected with the electron gun as disclosed in said Patent No. 2,176,199. In certain types of cathode-ray tubes no deflection plates are employed, for example, in the electromagnet deflection type tube. The return circuit for the secondary electrons from the floating screen is then by way of the second anode, which may extend up to the immediate neighborhood of the screen, e. g. as a metal or graphite coating on the inside wall of the conical part of the glass wall as is well-known in the art.

In accordance with the invention the coating which forms the screen 5 comprises a suitable fluorescent material in powdered form mixed with another material which has the property of acting as a binder during the application of the coating, and also may have the property of increasing the emission of secondary electrons from the coating when the latter is bombarded by the beam. In general there have been used three different methods of applying the fluorescent material to the cathode-ray tube wall. First there is the liquid settling method wherein the powdered fluorescent material is poured into the tube in the form of a liquid suspension and the powdered material is allowed to settle by gravity on to the end of the tube. Obviously this method is very slow and sensitive to mechanical vibration, particle size, viscosity and other characteristics of the liquid vehicle. Second there is the air settling method wherein a thin layer of a binder material usually of the nitro cotton type, or a binder of the sodium or potassium silicate type having organic ingredients, is applied usuis properly dissolved in a liquid solvent and the solution is used as a vehicle to suspend the fluorescent powder. This suspension is then sprayed onto the surface provided for supporting the fluorescent screen, generally by means of a specially designed spray gun. With this third method considerable difliculty is encountered in clogging of thespray gun and the gun must I be cleaned before and after each spraying operation. The method according to the present invention overcomes the above-noted and other disadvantages of the various known methods.

I have found that the use of a nitro cotton binder, or of a binder of the potassium or sodium silicate type with organic constituents, is sensitive to comparatively small elevations of temperature. For example during application they have a tendency to darken or become-cloudy at a temperature of the order of 250 degrees C. when exposed to air. In order to facilitate drying of the hinder, the glass support is usually heated during the application of the coating with the result that it is necessary to control the temperature -with extreme accuracy to avoid darkening of the screen. Furthermore the alkali metal content of certain of these binders causes a chemical reaction at the surface of the glass support, and thus destroys the transparency thereof after the fluorescent material has been deposited. In accordance with the present invention I employ a binder which is substantially entirely free from organic constituents and also from alkali metals or other constituents which react undesirably with the glass support. This binder can be heated in air to any desired temperature without materially effecting the transparency of the glass support.

Preferably the binder consists of a water or alcoholsolution of a dried hydrolized metallic salt. For example, but without limitation thereto, this salt may consist of dried hydrolized aluminum nitrate prepared in accordance with the process described in detafl in Russell E.

Pahnateer Patent No. 2,179,453, granted Novemher 7, 1939. In general, the binder is prepared by adding to an acid, for example nitric acid, a metal for example aluminum, in respective molar proportions in excess of those necessary to produce a normal or neutral salt of the metal and acid. Thus the aluminum in comminuted form may, as described in said Patent No. 2,179,453, be treated with nitric acid in the proper amount so that there results a viscous solution having the apparent composition of a hydrolized aluminum nitrate. Upon continued evaporation of the liquid, it becomes correspondingly more viscous and finally assumes a clear non-crystalized solid state. This clear solid upon further drying over a water bath becomes brittle and cracks so that it may be readily comminuted or reduced to powder form, this powder being readily soluble in water or in an alcohol to produce a viscous binder liquid.

This binder liquid may be used inany of the usual methods of applying the fluorescent material to the glass support. Preferably however the fluorescent material in powder form is mixed with the binder to the proper consistency or viscosity and the mixture is sprayed on to the wall I. I have found that this mixture, because of its viscosity -being easily controllable, can be used with known types of spray guns, for example as illustrated in U. S. Pat. No. 1,956,637 without much trouble from clogging of the spray nozzle. This feature is very important in the manufacture of cathode-ray tubes because if the spray been sprayed on to the wall 4 the latter may be heated in the air to as high a temperature as is desired since increase of temperature merely breaks the binder down into A1203 and some nitrogen compound the latter being removed either by a current of air or by evacuation. The breakdown of the binder by heat is part of the processing of the cathode-ray tube, and it may be advisable to heat the bulb, after the application of the coating, for several hours at 200 degrees C. to 300 degrees C. before the various electrodes 6 to II are sealed into the bulb. The gaseous components released by this heat treatment escape from the bulb, leaving A1303 distributed very uniformly over the screen.

This even distribution of the aluminum oxide over the screen increases its fluorescent efl'iciency when bombarded with electrons. It has been found possible to increase the fluorescent efliciency of fluorescent screens, even where these screens are attached to the glass by other methods than those described, provided A1203 is distributed evenly over the screen together with the fluorescent material. The explanation of this phenomenon is probably that at high voltages,

. the secondary emission of the screen is increased by the A1203 to such an extent that the potential of the floating screen never becomes substantially lower than that of the last accelerating electrode, for example electrode 9. In other words if the emission of secondary electrons from a floating screen is smaller than the amount of the electrons bombarding the screen, the screen would assume a lower potential, thereby reducing the velocity of the impinging electrons considerably. This decrease of speed of the bombarding electrons decreases the screen efficiency, as is readily understood. Regardless of the correctness of the above-described theory, it has been found that the screen efliciency actually can be improved by the addition of A; to the fluorescent material.

Another advantage of using the above-described method and materials is that with the prior methods and materials certain screens require washing during processing. I have found that using the methods and materials in accordance with the present invention this washing operation is not necessary. If, however, washing should be desirable, it has been found that screens prepared and attached as described herein are more durable under the washing process than screens deposited with prior binders or vehicles. Furthermore, since the binder does not contain any alkali metals the heating of the coating does not cause undesirable chemical rei gaseous filling such as neon,

action with the glass 4 and there is substantially negligible tendency for the glass to change its transparency, and if desired the coating can be washed off from the glass any desired number of times without impairing the transparency thereof.

While in the preceding description aluminum has been mentioned as the metal constituent of the binder and the resultant oxide it will be understood that other metallic salts or nitrates may be employed in preparing the binder, for example salts of magnesium, aluminum, beryllium, titanium, zirconium. Likewise while metallic aluminum has been described in preparing the binder, it will be understood that combinations of aluminum with any one or more of the metals such as magnesium, beryllium, titanium or zirconium may be employed.

Instead of preparing the binder in the manner described above, it may be prepared by treating an aqueous solution of a metallic nitrate such as the normal or neutral salt with a quantity of the powdered metal of the salt .in the manner described in detail in Russell E. Palmateer Patent No. 2,179,381, granted November 7, 1939.

It will be understood that after the coating 5 has been applied, the various electrodes may be sealed into the tube which is thereupon evacuated, and if desired, provided with a suitable argon or the like as is well-known in the cathode-ray tube art. It will also be understood that while the special mixture of binder and fluorescent material is preferably applied by spraying it can also be applied by flowing, brushing or painting.

Various changes and modifications may be made in the materials and proportions thereof without departing from the spirit and scope of the invention. For example, while a water or an alcohol solution of the binder has been mentioned, it will be understood that the binder may be dissolved in a mixture of water and alcohol. The expression hydrolized metal nitrate as employed in the claims is intended to refer to a nitrate wherein the metal and nitrate are present in substantially equivalent atomic proportions.

What I claim is:

1. The method of attaching a fluorescent coating to a support which includes the steps of hydrolizing a. solution of a nitrate of a metal of the group consisting of magnesium, aluminum, beryllium, titanium and zirconium until the nitrogen and the metal of the salt are present in substantially equal atomic proportions, drying the solution and redissolving it to form a tacky viscous binder, applying the said binder and the powdered fluorescent material to the support, and drying the mixture on the support.

2. The method according to claim 1 in which the binder is applied to the support and then the fluorescent material in powdered form is applied to the binder.

3. The method according to claim 1 in which the binder is first applied to the support and then the powdered fluorescent material is applied to the binder by air settling.

4. The method according to claim 1 in which the binder is in the form of an alcohol and water solution.

5. The method according to claim 1 in which the powdered fluorescent material is mixed with the binder prior to application to the support.

6. The method according to claim 1 in which the dried salt is dissolved in water.

'7. The method according to claim 1 in which the dried salt is dissolved in alcohol.

CHARLES H. BACHMAN. 

